Ornithine Aminotransferase Deficiency in Gyrate Atrophy: Gyrate atrophy (GA) is a blinding, hereditary degenerative disease of the retina and choroid of the eye characterized by a generalized deficiency in the mitochondrial enzyme, ornithine aminotransferase (OAT). Using the OAT cDNA which we had characterized before, we have established the presence of an OAT gene family and mapped the functional OAT and other OAT-related gene sequences to chromosomes 10 and X, respectively. Restriction fragment length polymorphisms were found in the functional OAT gene sequence and in the OAT- related gene sequences on the X-chromosome which may potentially show a linkage to X-linked retinitis pigmentosa (XLRP) since the locus of these sequences is identical to that of the XLRP linkage marker L1.28. Analysis of the OAT gene, mRNA, and protein in GA patients identified a case with a partial heterozygous deletion of the OAT gene, no OAT mRNA, and essentially undetectable OAT protein. This finding is the first real demonstration of the OAT defect in GA at the gene level and establishes the molecular basis of the genetic defect present in GA. In order to determine whether our OAT cDNA clone contains all of the sequences necessary for expression of active OAT, we have also constructed a mammalian expression clone containing the OAT cDNA and expressed it in mouse fibroblasts. Ability to express OAT using our cDNA clone in mammalian cells opens up the possibility of considering a gene replacement therapy for GA. Hereditary Retinoblastoma: Hybrids between Y79 retinoblastoma and NIH3T3 cells were previously shown to be non-malignant, confirming the recessiveness of retinoblastoma. Variants were isolated among the hybrids that show reversion to malignant phenotype, suggesting that additional human gene(s) besides the chromosome 13 gene may be important in the suppression of malignancy in these cells and that progressive loss of these human genes from the hybrids may result in malignant reversion. Hybrids were also made between Y79 retinoblastoma cells and normal human fibroblasts, and shown to be non-malignant, confirming previous results.